Understanding the cellular and molecular mechanisms that regulate the generation of oligodendrocytes, the myelinating cells of the vertebrate central nervous system is essential both to establish a comprehensive vision of neural development and to effectively generate new therapeutic approaches towards demyelinating diseases. In the last funding period we have made substantial gains in defining the location and molecular signaling that regulates the appearance of oligodendrocyte precursors and their dispersal in the developing spinal cord. In the current proposal we outline a series of approaches that are designed to further our understanding of the cellular and molecular interactions that promote later stages of oligodendrocytes development and to relate that information to the regulation of successful myelin repair in the adult CNS. Building on new approaches and recent preliminary data we outline 3 specific aims that investigate distinct but related aspects of oligodendrocyte development. In the first aim we will test the hypothesis that mature MBP+ oligodendrocytes influence the timing and success of spinal cord myelination as the direct producer of myelin and through feedback signals to OPCs. In addition we will determine whether a local postnatal depletion of MBP+ oligodendrocytes compromises myelin repair in the same region of the adult following a second insult. In the second aim we will characterize the role of GFAP+ astrocytes in the generation of the spinal cord oligodendrocyte lineage and myelination and the third aim will address the role of both mature oligodendrocytes and astrocytes in the control of remyelination in the adult spinal cord. These studies take advantage of a novel approach developed in the laboratory during the last funding period in which we are able to selectively eliminate distinct populations of mature oligodendrocytes and astrocytes in precise locations at any point in the developing and adult CNS. To accomplish this we have generated a series of transgenic animals in which we express an inducible caspase 9 (iCP9) off cell type specific promoters. Induction of the iCP9 through cross-linking with a small molecule variant of the cell and tissue permeable FK506 stimulates apoptosis specifically in the targeted population of cells. These animal models provide us with a unique opportunity to assess the consequences of selective cell loss on myelination and myelin repair in the intact vertebrate CNS. The data generated during the course of these studies will provide new directions for the development of therapies for demyelinating diseases such as multiple sclerosis.